This invention relates to a current source circuit in which bipolar transistors are arranged such that the ratio of the output current to the input current can be precisely set to a desired value.
U.S. Pat. No. 3,320,439 discloses a circuit which is a bipolar IC (integrated circuit), as shown in FIG. 1, which converts an input current into an output current of a desired small value. In the circuit of FIG. 1, if the input current I.sub.1 is 100 .mu.A and the output current I.sub.2 is 0.1 .mu.A, the resistance of resistor R, which is given as (V.sub.T /I.sub.2).multidot.ln(I.sub.1 /I.sub.2), is 1.8 M.OMEGA., V.sub.T being the thermionic voltage. With the present bipolar IC techniques, however, it is difficult to realize a resistance in excess of 1 M.OMEGA. precisely. Therefore, the desired output current I.sub.2 cannot be obtained with high precision.
FIG. 2 shows another well-known circuit. In the circuit of FIG. 2, if a current of 100 .mu.A is directed to the emitter of the bipolar transistor and the grounded emitter current amplification factor .beta. is 100, the base current I.sub.B =(1/.beta.)I is 1 .mu.A. The precision of this base current I.sub.B, however, is inferior, as it depends upon the current amplification factor .beta.. According to current bipolar IC technology, the current amplification factor .beta. ranges approximately from 100 to 500. Therefore, it has been very difficult to obtain a current source circuit which can provide an output current of a small value, typically less than one microampere, using the conventional bipolar IC technology.
To overcome this deficiency, the inventor has developed a current source circuit which can provide a small current (less than one microampere) with high precision. This circuit has a basic construction as shown in FIG. 3. However, the ratio of the output current I.sub.0 to the input current I of this current source circuit, i.e., the amplification factor of the circuit, depends upon temperature. Therefore, the precision of the output current is still not perfectly satisfactory. The circuit of FIG. 3 will be described in order to facilitate the understanding of the invention.
Referring to FIG. 3, a current source of input current I and transistors Q1 and Q2 are provided in series between a high potential point (+) (for instance, at 10 V) and a low potential point (-) (for instance, at ground potential). Another series circuit, which consists of a transistor Q3, a resistor R1 and a current source which provides a current of nI (n being a positive integer), is provided between the high potential point (+) and the low potential point (-). Further, a transistor Q4 is connected between the low potential point (-) and an output terminal OUT, from which the output current I.sub.0 is provided. The transistors Q1 and Q2 each have their collector and base connected to each other. The collector of the transistor Q1 is connected to the base of the transistor Q3. The junction between the transistor R1 and the current source of the current nI is connected to the base of the transistor Q4. The transistors Q1 to Q4 have respective emitter areas m1 to m4, which are set to be m1&gt;m3, m1&gt;m4, m2&gt;m3 and m2&gt;m4, that is, m3=m4=1=A and m1=m2=m=mA.
Assuming that the grounded emitter amplification factor .beta. of each of the transistors in FIG. 3 is sufficiently large, for the closed loop including the transistors Q1 to Q4 and resistor R1 there holds a relation EQU V.sub.BE (Q2)+V.sub.BE (Q1)=V.sub.BE (Q4)+nI.multidot.R1+V.sub.BE (Q3) (1)
where V.sub.BE is the voltage across the base-emitter path of the relevant transistor. Meanwhile, the base-emitter voltage V.sub.BE on each transistor and the collector current I.sub.C thereof are related as ##EQU1## where V.sub.T is the thermionic voltage, m is the ratio between the emitter area of each of the transistors Q1 and Q2 and the emitter area of each of the transistors Q3 and Q4, and I.sub.S is the reverse biased saturation current of the transistors. Substitution of the equation (1) into the equation (2) yields an equation ##EQU2## Since m1=m2=m and m3=m4=1, from the equation (3) we obtain ##EQU3## From the equation (4) we obtain ##EQU4## The equation (5) means that the output current I.sub.0 from the transistor Q4 is determined by the transistor emitter area ratio m, the current source current ratio n and the resistance of the resistor R1. The resistance of the resistor R1 need not be of a high value as mentioned in connection with FIG. 1. Thus, the current source circuit can be readily obtained as a bipolar transistor integrated circuit. In addition, a small current of the order of 0.1 .mu.A can be obtained with high precision. Further, the magnitude of the output current I.sub.0 can be readily adjusted by suitably selecting the resistance of the resistor R1. In the circuit of FIG. 3, the ratio of the output current I.sub.0 to the input current I, i.e., the amplification factor G of the circuit, is given as ##EQU5## As is obvious from the equation (6), the current amplification factor G depends upon temperature due to the thermionic voltage V.sub.T.